12а-3в÷12а+3в при= -3/4 в =-5/6

To simplify the expression, we'll substitute the values given:

12a - 3b ÷ (12a + 3b) = -3/4

Let's substitute b = -5/6:

12a - 3(-5/6) ÷ (12a + 3(-5/6)) = -3/4

12a + 5/2 ÷ (12a - 5/2) = -3/4

Now, to simplify further, we'll get rid of the fractions by multiplying both sides by the common denominator of 4:

4(12a + 5/2) ÷ 4(12a - 5/2) = -3

48a + 10 ÷ 48a - 10 = -3

Now, we can solve for a:

48a + 10 = -3(48a - 10)

48a + 10 = -144a + 30

48a + 144a = 30 - 10

192a = 20

a = 20/192

a = 5/48

Therefore, the simplified expression is 5/48.